Synaptic activity represents not only a mode of interneuronal communication, but may also influence the development of synaptic circuits. A variety of studies have shown that neuronal morphology and circuitry in the cerebellum are dependent on extrinsic, cell-cell interactions, which makes the cerebellum an excellent model system to study the affects of changes in synaptic activity on neuronal form and function. The purpose of this multidisciplinary project is to analyze the role of neuronal and synaptic activity in the development of synaptic circuits using the mouse cerebellum as a model system. To test this hypothesis, the cerebella of wild type neonatal mice and neonatal mice with the Purkinje cell genetic mutation, lurcher, will be chronically exposed to tetrodotoxin (TTX) to block neuronal activity or excitatory amino acid (EAA) antagonists to block excitatory synaptic transmission. The EAA antagonists include; kynurenic acid, a non-specific EAA receptor antagonist; CNQX, a non-NMDA receptor antagonist, and AP-5, NPC 12626, MK-801, and 7-chlorokynurenic acid, NMDA receptor antagonists. Each drug will be dissolved in a plastic polymer, elvax, that slowly releases the drug over time. Thin sections of drug- treated or control elvax will then be implanted over the developing cerebellum of neonatal mice. The proposed studies will focus on the effects of chronic TTX or EAA receptor antagonist treatment on the development of the cerebellar mossy fiber-granule cell-Purkinje cell circuit. Electrophysiological and anatomical techniques will be used to characterize changes in this circuit due to the experimentally induced changes in neuronal or synaptic activity. The drug-treated mice will be analyzed in four different experimental protocols designed to address the following questions: 1) does blocking neuronal activity alter the responsiveness of Purkinje cells to EAA agonists and the density of EAA receptors in the developing cerebellum; 2) what is the effect of blocking neuronal activity on the development of spinocerebellar mossy fiber columns; 3) how does blocking neuronal activity affect Purkinje and granule cell survival; and 4) how does blocking neuronal activity affect dendritic morphology of cerebellar neurons? This study is directed towards understanding how functional neuronal circuits are established in development and how synaptic activity affects neuronal plasticity.